The correct answer is: a) Bhagalpur.
Bhagalpur is a city in the Indian state of Bihar. It is the headquarters of Bhagalpur district and Bhagalpur division. The city is located on the banks of the Ganges River, about 200 kilometers (120 mi) north of Patna, the state capital.
Bhagalpur is known for its silk production and educational institutions. The city is home to the National Institute of Technology, Durgapur, the Indian Institute of Technology (BHU), and the Bhagalpur University.
Nalanda is an ancient city in the Indian state of Bihar. It was once a major center of learning and Buddhism. The city is now a UNESCO World Heritage Site.
Rajgir is a city in the Indian state of Bihar. It is located about 100 kilometers (62 mi) south of Patna, the state capital. The city is known for its Buddhist temples and monasteries.
Vaishali is an ancient city in the Indian state of Bihar. It was once the capital of the Vajji confederacy. The city is now a UNESCO World Heritage Site.
Bhagalpur is the only city among the given options that is known for its silk production and educational institutions.